1. Field of the Invention
The present invention relates to an electrical light emitting device used in displays, backlighting for liquid crystal displays, light sources for indicators and optical communications or the like.
2. Description of the Related Art
An electroluminescent (abbreviated as EL hereafter) device is a light emitting device utilizing the light emitting phenomenon of solid fluorescent material under an application of an electric field, i.e. electroluminescence. At the present time, inorganic EL devices using inorganic material as the light-producing body have been successfully developed and used as EL devices in backlighting of liquid crystal displays, flat panel displays, and the like. However, the inorganic EL devices have such inherent defects as needing a high voltage of 100 V to 200 V for luminescence of the devices, difficulties in realizing color displaying or the like.
On the other hand, efforts in research of EL devices formed of organic material have been made for a long time, but such efforts have not materialized in commercialization of viable products due to inferior performance of the organic EL devices when compared with the inorganic EL devices. However, as appears in Appl. Phys. Lett. 51(1987) 913 by C. W. Tang and S. A. VanSlyke, an organic thin film EL device was proposed by Tangs and others in 1987.
More specifically, an organic EL device formed of two layers of organic material, a hole carrier layer and a light emitting layer, was able to operate under a DC voltage and produce high brightness luminance exceeding 1000 cd/m.sup.2 with an application of a low voltage of less than 10 V. Since that time on, R&D activities for a successful commercialization of similar organic thin film EL devices of a multi-layer structure have been carried out vigorously.
In connection with the foregoing organic EL devices, devices of various luminescent wavelengths including blue light that were difficult to produce in a form of inorganic EL devices have been realized through changing luminescent material and layer structures, thereby arousing interests in applying such EL devices to various light emitting products and full color displays.
However, due to the existence of many non-light-emitting spots on the luminant surface of the organic thin film EL devices available at present, it is rather difficult to obtain devices of uniform surface luminance, and also on account of a markedly increased number of non-light-emitting spots and a noticeable change in the brightness reduction observed over a long period the devices are kept long on the shelf or operated continuously, the EL devices that achieved a practical level in performance and usage have been not yet available.
The causes of the foregoing non-uniform luminance and performance deterioration over time may be attributed to lack of a sufficient adherence between the hole injection electrode and the organic thin film layer.
An effort to improve the adherence of the hole injection electrode to the organic thin film layer has been made by roughening the surface of the hole injection electrode through reverse sputtering or the like.
Although uniform luminance is improved for the initial period of operation through the foregoing attempt, such defects as an increase in non-light-emitting spots, when the device is kept long on the shelf and operated continuously, and a reduction in brightness, when the device is operated continuously, have not been corrected.
This is considered to be attributed to the poor adherence of the organic thin film layer, which is an organic substance, to the hole injection electrode, which is an inorganic substance, and also to peeling off of the organic thin film layer from the hole injection electrode when the device is kept long on the shelf or operated continuously because of an excessive difference in thermal expansion coefficient between both.
The object of the present invention is to provide an organic thin film EL device, and its manufacturing method, having uniform luminance and achieving a long life with a reduced change in performance over a long time period.